Electrical-treeing phenomena which occur in polymers such as low-density polyethylene (LDPE) and crosslinked polyethylene (XLPE) have been under study for many years. Several mechanisms have been proposed to explain electrical treeing in insulation materials subjected to high electric fields. Among these are: (a) fatigue cracking due to Maxwell stress, (b) Joule heating that leads to thermal decomposition, (c) high field-induced impact ionization and (d) hot electrons that can break polymer bonds. However, none of these can explain adequately the gradual degradation that leads to electrical-tree initiation in cables subjected to stresses much lower than the breakdown strength of the polymeric insulation. Mechanism (a) cannot be responsible for tree initiation because mechanical stresses produced in polyethylene (PE) cables operating at working stresses are only a fraction of the tensile strength of the polymer. Mechanism (b) requires the preexistence of a cavity within which partial discharges (PD) can occur, but tests with needles in solids have shown that no initial void at the needle tip is required to start tree growth. Mechanisms (c) and (d) require that the charge carriers in the polymer gain large energies from the electric field. But since the mean free path of charges in PE is of the order of a few molecular radii (5-20 .ANG.), it is almost impossible for them to become hot enough to cause impact ionization or break bonds of the polymer chain. However, in high-voltage cables, gradual degradation that leads to electrical-tree initiation probably occurs at electrical fields much lower than the breakdown strength of the polymeric insulation. Thus, besides the fact that a stress concentration is always required, the initiation mechanism of electrical trees is not fully understood. Defects that are accidentally introduced into the polymer during cable manufacture become points of high local stress and reduce insulation performance. Such points of high electrical stress are usually simulated in the laboratory by molding needles into the polymer.
To overcome the problem of electrical treeing, several solutions have been proposed thus far. For instance, McMahon U.S. Pat. No. 4,206,260 proposes using LDPE or XLPE insulation with an amount of an alcohol of 6 to 24 carbon atoms. Maloney U.S. Pat. No. 3,499,791 discloses a polyethylene insulation containing an inorganic ionic salt of a strong acid and a strong Zwitter-ion compound. Kato et al, U.S. Pat. No. 3,956,420 discloses insulation comprising a polyolefin, a ferrocene compound and a substitute quinoline compound. Additionally, a small amount of polyhydric alcohol, dispersant, surfactant or unsaturated polymer or mixture thereof is used. MacKenzie, Jr. U.S. Pat. No. 3,795,646 recommends the use of a silicone fluid in a crosslinked polyethylene composition.
Shimizu et al (IEEE Trans. Electr. Insul. El-14, 256 (1979) have reported that light is emitted at needle tips in LDPE subjected to highly divergent fields at a cryogenic temperature (liquid nitrogen). Bamji et al. (Annual Report of 1982 Conference on Electrical Insulation and Dielectric Phenomena. IEEE Service Center, Piscataway, N.J., p. 592) have discovered similar emissions at room temperatures. This light has been attributed to electroluminescence (EL) caused by charge injection into the polymer from the metallic point.